[NEXT][CLEANUP] Remove legacy execution context and KGRAM residue

[remiceres]

Parent:
#543

Depends on:
#549

Goal

Remove the remaining legacy contracts inherited from the copied KGRAM code.

Scope

• Reuse and simplify the existing BindingContext. Do not introduce another binding abstraction.
• Remove legacy Binding, ASTExtension, Metadata, visitors, reporting hooks, and exceptions when they are not needed by the supported runtime.
• Remove remaining references to old KGRAM types, including fully qualified and static references.
• Prune copied methods and fields proven unreachable by the audit.
• Keep neutral extension points only when they have an identified next consumer.

Acceptance criteria

• No retained KGRAM next contract exposes a legacy SPARQL or KGRAM type.
• Every removed compatibility surface is covered by the audit.
• The native runtime corpus remains green.

Estimate: 8–15 development days.